1. Field of the Invention
The present invention relates to monitoring of EEG signals. The invention more particularly relates to a personal wearable EEG monitor adapted to be carried at the head of a person. The EEG monitor comprises an EEG sensor part having electrodes arranged on the skin surface of a person for measuring one or more EEG signals from said person. The EEG monitor further comprises an EEG signal analyzer adapted for having an EEG signal transferred from the EEG sensor part, and adapted for monitoring the EEG signal from the person wearing the device. The invention further relates to a method for monitoring EEG signals of a person.
Personal wearable EEG monitors are known for surveillance of EEG in order to detect imminent seizures, but could also be applied for long term EEG recording.
2. The Prior Art
Such personal wearable EEG monitors are known from WO 2007/150003 describing a system for long term EEG monitoring with implanted electrodes.
WO 2006/066577 describes a personal monitoring device for detecting the onset of hypoglycemia by analyzing an EEG signal obtained through implanted electrodes.
WO 2007/047667 describes EEG monitoring partly by application of an electrode in the ear canal. The application of auditory evoked potentials is also described.
It has been found that wearable personal devices for long term measurements of EEG-signals can be located in the region behind or in the ear of the user with several advantages. This location is ideal for physiologic, cosmetical and mechanical reasons.
A measurement of the EEG signal in the ear canal has the advantage of being protected against external electrical fields since the ear canal extends into the head which will shield the EEG electrodes partly. It is possible to obtain a very good fit between an earpiece holding the electrode and the ear canal, and thereby the contact between the skin and the electrode becomes less sensitive to movements and skin strain. Further to this, the ear itself, or part of it, may be used for attachment of the device. Many EEG signals are also available from the ear region.
Other examples of wearable EEG monitors could be hearing aids with EEG-feedback (e.g. the hearing aid is in some way adjusted according to information extracted from an EEG signal) and personal health monitoring devices. Examples of personal health monitoring devices could be hypoglycemia warning devices for persons with diabetes, and seizure warning devices for persons with epilepsy. Also continuous monitoring of the EEG signal for diagnostic or research purposes may be relevant.
The requirements and trade-offs between different characteristics for electrodes for EEG measurements in wearable personal devices are different from those for electrodes for clinical use, e.g. short term EEG monitoring of a patient in a hospital. Typical requirements for electrodes in wearable personal devices are, that they must be easy to put in place, they should not exert any stress on the skin (e.g. no strain of the skin), they must be comfortable and small (e.g. the size of a hearing aid), they must be cosmetically acceptable, addition of gel between skin and electrode should be avoided (i.e. dry electrodes), and in general no skin preparation should be necessary. These requirements compromise the signal acquisition properties and the reliability of the electrodes, as the requirements will make it more difficult to obtain a good electrical contact between skin and electrode. Thus electrodes designed for such devices have typically much larger impedances (e.g. in the hundreds of kilo Ohm range), larger variations in impedances, and are less reliable than electrodes for clinical use.
Traditionally electrodes for electrical bio-potential measurements, such as EEG, are validated by measuring the electrical impedance between two or more electrode elements. This method is feasible for clinical use and for electrodes with reasonably low impedances, e.g. less than a few hundred kilo Ohms. Measurement of electrical impedance has now been found not to be sufficiently reliable for electrodes with very large electrical impedance. This is because measuring the electrical impedance will only reveal if there is an electrical connection between two electrodes, but not if the electrodes are measuring an EEG signal. An electrical connection may just be due to a layer of dirt on the part holding the electrodes.
In long term monitoring of EEG-signals in wearable personal devices there is a need for validating the EEG signal measured by the electrodes, and for the reasons mentioned above there is a need for an alternative method to the electrical impedance method. The electrode validation must be easily performed by the user. Furthermore the electrode validation should preferably be an integrated capability of the device.